As is well known, halogen compounds and phosphorus/nitrogen compounds exhibit a conspicuous flame retardant effect, and are often used as a flame retardant agent of a thermoplastic resin.
Also, the above described flame retardant agent is essential in a recycling process in which used PET (polyethylene terephthalate) bottles are crushed, washed, and made into source materials again to form polyester fibers. Currently, halogen flame retardant agents made of a bromine compound or a chlorine compound are widely used.
Also, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-263188 discloses ammonium dihydrogen phosphate as one of the choices for a phosphorus/nitrogen flame retardant component.
The aforesaid conventional halogen flame retardant agent is excellent in a flame retardant effect; however, it raises a problem in that the fibers and the like that are made flame retardant by the halogen flame retardant agent give rise to a cause of generating harmful gases or dioxins at the time of burning.
Also, non-halogen flame retardant agents such as ammonium dihydrogen phosphate produce an effective flame retardant power when added in an amount of 15% by weight or more to the recycled PET resin. However, with this amount of addition, there is raised a problem in that the physical properties of the PET resin decrease and a problem of giving negative effects in a process of forming the PET resin into fibers by deterioration in the physical property.
Inherently, a thermoplastic resin such as polyethylene terephthalate has a poor compatibility with mingled foreign substances, and hence the molecular weight decreases because the molecular chains are cut by hydrolysis or thermal decomposition. Due to this physical property deterioration, the thermoplastic resin loses its viscosity and hence cannot be drawn and extended in a thread form.
Therefore, the present inventors have repetitively conducted sample-making experiments in order to realize a technical goal of providing a flame retardant agent for a thermoplastic resin, which enables a thermoplastic resin containing the flame retardant agent to keep its viscous property, to be made into fibers readily, and to exhibit flame retardant effects even when ammonium dihydrogen phosphate is contained in the flame retardant agent as a non-halogen flame retardant agent, as well as a flame retardant resin composition kneaded and molded after addition of the flame retardant agent. In the process of the experiment, the present inventors have conceived the following idea. Namely, when kneading is carried out after a flame retardant agent is added into a thermoplastic resin, a flame retardant agent having poor dispersibility increases the ratio of the parts where the flame retardant agent is absent, whereby the flame retardant effects are hardly exhibited, and the product will be liable to catch fire. Therefore, in order to ensure a certain flame retardancy by using a flame retardant agent having poor dispersibility, the flame retardant agent must be added in a large amount. Addition of a large amount causes decrease in the melt viscosity of the thermoplastic resin or decrease in the strength, thereby to invite decrease in the fiber-forming property or spinnability. Therefore, in order to impart flame retardancy to a thermoplastic resin having an affluent flammability, it is desirable that the flame retardant agent is dispersed as homogeneously as possible. Further, as the dispersibility is improved more and more, the intrinsic function of the flame retardant agent is exhibited to the maximum extent, and a large flame retardant effect is obtained with addition of a small amount. Further, the improvement in the dispersibility contributes not only to the improvement in the flame retardancy but also to the homogenization of the physical property of the fibers, thereby to decrease the defects at which the stress concentration occurs, leading to an improvement in the strength. On the basis of this idea, the present inventors have further made numerous experiments in a trial-and-error manner for making the PET resin flame retardant by conceiving the use of the used PET bottles as the thermoplastic resin for re-use as fibers.
As a result of the experiments, the present inventors have confirmed that, since ammonium dihydrogen phosphate (NH4H2PO4) releases ammonia at around 170° C. to decrease the pH value from 4.7 to 2.5, a flame retardant agent made only of the ammonium dihydrogen phosphate deteriorates the PET resin to decrease the IV value (intrinsic viscosity), and have obtained a remarkable knowledge that, when ammonia is volatilized in advance by allowing potassium hydroxide (KOH) to react with ammonium dihydrogen phosphate in order to prevent the decrease in the IV value, the PET resin having the flame retardant agent added thereto satisfies an IV value that enables fiber forming, thereby achieving the above described technical goal.